1. Field of the Invention
The present invention relates to a method for producing an electrode for a lithium secondary battery.
2. Related Art
In recent years, research and development of lithium secondary batteries have been actively made. The battery performance of lithium secondary batteries, such as charge-discharge voltages, charge-discharge cycle life characteristics or storage characteristics, depends largely on the electrodes used. Therefore, improvement of active materials used for the electrodes has been attempted to enhance the battery performance.
A battery with high energy densities both per weight and per volume can be obtained by using lithium metal as a negative active material. This battery however has a problem that the lithium deposited on charge grows into dendrite, causing an internal short-circuiting.
To solve the above problem, there is reported a lithium secondary battery using any of aluminum, silicon, tin and the like, which are electrochemically alloyed with lithium during charging, as an electrode (Solid State Ionics, 113-115, p. 57 (1998)). Among the above metals, silicon, having a large theoretical capacity, is particularly promising as a battery negative electrode capable of providing a high capacity. Various secondary batteries using silicon as the negative electrode have been proposed (Japanese Patent Laid-Open No. 10-255768). However, this type of alloy negative electrode fails to provide sufficient cycle characteristics because the alloy as the electrode active material itself is pulverized during charging and discharging, resulting in reducing the current-collecting characteristics.
There have been proposed an electrode for lithium secondary batteries using silicon and the like as an electrode active material that exhibit a good charge-discharge cycle characteristics (International Patent Laid-Open WO01/31720A1 etc.), in which a microcrystalline or amorphous thin film is formed on a current collector by a thin-film forming method such as a CVD or sputtering method.
In the electrodes for lithium secondary batteries as described above, it is known that a component of the current collector diffuses into the thin film of active material, and that this serves to maintain adhesion between the current collector and the thin film of active material and thus improve the charge-discharge cycle characteristics. Therefore, in order to obtain excellent charge-discharge cycle characteristics, the interface between the current collector and the thin film of active material is preferably formed under control under optimal conditions. However, the thin film formed on the current collector must have some degree of thickness to be used as an active material. If the thin film of active material is formed under optimal thin-film forming conditions as described above, a long time is required for formation of the thin film, and thus high productivity is not obtained. As another problem, the current collector becomes hardened if it is exposed to high temperature for a long time during the thin film formation. This causes difficulty of changing the shape of a current collector in battery production.
An object of the present invention is to provide a method for producing an electrode for a lithium secondary battery, capable of depositing a thin film of active material on a current collector at a high film formation rate without deteriorating the mechanical properties of the current collector.
The present invention is directed to a method for producing an electrode for a lithium secondary battery, the electrode having an active material in the form of a thin film composed of an interface layer formed on a current collector and an active material layer formed on the interface layer. The method includes the steps of: depositing the interface layer on the current collector by sputtering; and depositing the active material layer on the interface layer by vapor evaporation.
Sputtering is a method for depositing a thin film by sputtering an active species generated from plasma against a current collector as a substrate. Therefore, the resultant thin film can provide a good interface with the current collector, and thus the adhesion of the thin film to the current collector is improved. Vapor evaporation is a method enabling deposition of a thin film at a film formation rate higher than that of the sputtering. According to the present invention, an interface layer is formed by sputtering, and then an active material layer is formed by vapor evaporation. Therefore, while the interface having good adhesion with the current collector can be formed, the active material thin film can be formed at a high film formation rate. In this way, it is possible to efficiently produce an electrode for a lithium secondary battery having a high charge-discharge capacity and excellent charge-discharge cycle characteristics.
The vapor evaporation method employed in the present invention is not specifically limited as long as it has a film formation rate higher than the sputtering. Vacuum vapor evaporation such as electron beam vapor evaporation and other vapor evaporation methods may be employed.
The material used as the active material in the present invention is not specifically limited as long as it can store and release lithium. However, a material storing lithium by being alloyed with lithium is preferred. Examples of such a material are silicon, germanium, tin, lead, zinc, magnesium, sodium, aluminum, potassium and indium. Among them, silicon is particularly preferred due to its high theoretical capacity. As silicon, amorphous or microcystalline silicon is preferred.
In the case of forming the thin film of active material on both surfaces of the current collector, the interface layer and the active material layer are preferably formed on both surface of the current collector.
According to the present invention, the thickness of the interface layer is preferably 1 xcexcm or less. If the thickness of the interface layer is larger, the thickness of the active material layer, which can be formed at a high film formation rate, is relatively smaller. This decreases the film formation rate as a whole, and thus may result in failure of attaining the object of the present invention of obtaining a high film formation rate. Also, the thickness of the interface layer is preferably 0.01 xcexcm or more. In view of the above, the thickness of the interface layer is preferably in the range of 0.01 to 1 xcexcm.
According to the present invention, both the sputtering for forming the interface layer and the vapor evaporation for forming the active material layer are performed under an evacuated atmosphere. Therefore, the formation of the interface layer and the formation of the active material layer are preferably performed successively in an evacuated atmosphere. By performing the successive formation without exposure to the air atmosphere, introduction of impurities into the layers is prevented. Thus, the interface layer and the active material layer are preferably formed within a same thin-film forming apparatus. Preferably, in such a case, the current collector is moved to respective positions for the formations of the interface layer and the active material layer.
In the case of forming the thin film of active material on both surfaces of the current collector according to the present invention, the formation of the interface layer and the active material layer on one surface of the current collector and the formation of the interface layer and the active material layer on the other surface of the current collector are preferably performed successively under an evacuated atmosphere.
The present invention is directed to a method for producing an electrode for a lithium secondary battery, which is applicable to both a negative electrode and a positive electrode. When the material such as silicon described above is used, the electrode is generally used as a negative electrode.
The current collector used according to the present invention is preferably made of a metal incapable of being alloyed with lithium. Examples of such a material are copper, copper alloy, nickel and stainless steel. The current collector may also be composed of a stack of two or more types of these materials.
According to the present invention, the interface layer and the active material layer are generally made of the same type of material, but may be made of different materials.